The disclosure of producing transparent and thermally-opacifiable photosensitive glasses is founded in such patents as U.S. Pat. Nos. 2,515,936, 2,515,943, and 2,651,145.
Thus, U.S. Pat. No. 2,515,936 refers to a "silicate glass containing, on the oxide basis by weight, 0.05 to 0.3% of silver computed as AgCl, and CeO.sub.2 in an amount up to 0.05%". The specification refers specifically to glass batches equivalent to about 75% SiO.sub.2, 15% R.sub.2 O, and 10% RO. CeO.sub.2 is stated to act as a sensitizing agent. The method contemplates exposing the glass to such shortwave radiations as ultra-violet radiations, X-radiations, and radioactive radiations and then heating the exposed glass to temperatures between about 470.degree.-500.degree. C. The coloring is due to the development of colloidal metal particles, e.g., silver, in the glass as a result of the radiation step followed by heat treatment.
U.S. Pat. No. 2,515,943 describes thermally-opacifiable, photosensitive glasses wherein NaF crystals constitute the opacifying phase. The base glasses consisted essentially of 55-75% SiO.sub.2, 12-18% R.sub.2 O, 2-12% Al.sub.2 O.sub.3, 1.8-2.4% F, 0.005-0.05% CeO.sub.2, and 0.001-0.01% Au. The glass is exposed to shortwave radiations and heated to a temperature within the range of about 100.degree. C. below the softening point of the glass to about 50.degree. C. above the softening point. Submicroscopic nuclei of colloidal gold are formed thereby. The nucleated glass is cooled below 520.degree. C. to cause the formation of alkali metal fluoride nuclei on the colloidal gold particles, and thereafter the glass is reheated to 520.degree.-570.degree. C. to cause the fluoride nuclei to grow into opacifying crystallites. The image resulting was normally of an opaque white although the occurrence of a pink tint was noted.
U.S. Pat. No. 2,651,145 discloses thermally-opacifiable, photosensitive glasses wherein an alkali metal fluoride comprises the opacifying phase. The base glasses consisted essentially of 55-75% SiO.sub.2, 5-18% Na.sub.2 O, 0-2% Li.sub.2 O, 0-13% K.sub.2 O, 2-12% Al.sub.2 O.sub.3, 1.8-3% F, 0.005-0.05% CeO.sub.2, and 0.0001-0.3% Ag, computed as AgCl. The glass was subjected to shortwave radiations and heated to a temperature within the range of about 150.degree. C. below the softening point of the glass to about 50.degree. C. above the softening point. Those steps resulted in the development of submicroscopic nuclei of colloidal silver. The nucleated glass was cooled below 500.degree. C. to form alkali metal fluoride nuclei on the particles of colloidal silver. The glass was subsequently reheated to a temperature not lower than about 100.degree. C. below the softening point of the glass to cause the fluoride nuclei to grow and form opacifying crystallites. By varying the shortwave radiation exposure time and the time and/or temperature of the first heating step, the coloration of the opacified article can be varied widely.
Although, as can be seen from the above, various colors could be produced in transparent and thermally-opacifiable (opal) glasses, control of the colors produced was so extremely difficult that little practical use could be made of the described practice. Therefore, the primary objective of the instant invention is to provide a method for producing colors in transparent and opal photosensitive glasses which permits the ready control of the colors produced.